This application corresponds to Japanese Patent Application No. 8-328543, filed on Dec. 9, 1996, which is hereby incorporated by reference in its entirety herein.
1. Field of the Invention
The present invention generally relates to choke coils and more particularly relates to a choke coil used for suppressing noise generated in or entering electronic equipment.
2. Description of the Related Art
Typically, since a common-mode choke coil has a slight leakage inductance component in the normal mode, this choke coil is effective against normal-mode noise, as well as against common-mode noise. If, however, the normal-mode noise is too strong, a normal-mode choke coil should be independently used to reduce such noise.
To address the above-described needs, the choke coil shown in FIG. 11 has been proposed as a coil for effectively suppressing both common-mode noise and normal-mode noise. This choke coil is formed of a magnetic bobbin 100, a pair of windings 105 and 106, and a magnetic core 110. The magnetic bobbin 100 has a core portion 101, and flanges 102, 103 and 104 which are respectively provided at both outer ends and at the center of the core portion 101. The windings 105 and 106 are wound around the core portion 101. One side of the magnetic core 110 is inserted into a hole 101a formed in the core portion 101.
In the choke coil configured as described above, a common-mode noise current flows into the pair of windings 105 and 106 so as to generate a magnetic flux in each of the windings 105 and 106. The sets of generated magnetic flux are combined and are converted into thermal energy in the form of eddy current loss in the magnetic core 110, and then attenuated. The common-mode noise is thus reduced.
Furthermore, a normal-mode noise current flows into the windings 105 and 106 so as to generate a magnetic flux in each of the windings 105 and 106. The generated magnetic flux is converted into thermal energy in the form of eddy current loss in the magnetic bobbin 100, and, as a result, then attenuated. The normal-mode noise is thus suppressed.
In the above known type of choke coil, since the windings 105 and 106 are wound around substantially the overall peripheral area of the core portion 101, the flange 104 is thickened (e.g., widened) in order to reliably establish an insulation between the windings 105 and 106. For example, the thickness of the flange 104 is set so that the distance L from one end of the winding 105 to one end of the winding 106 along the exposed surface of the flange 104 is approximately 3.2 mm. On the other hand, the heights T of the flanges 102, 103 and 104 of the bobbin 100 have heretofore not been a consideration in the design of these bobbins, and the heights T are generally set so as to have the same value.
In the conventional choke coil, the gap D between the forward end of the flange 104 and the magnetic core 110 is comparatively large, as illustrated in FIG. 12. Consequently, the magnetic flux generated due to the flow of the normal-mode noise current hardly passes through the magnetic core 110. This design therefore fails to use the magnetic core 110 to provide a magnetic flux path formed by the flow of the normal-mode noise current. As a result, the above-described conventional choke coil obtains a normal-mode inductance component having low efficiency.